Fireplaces have been used for many years for heating, cooking, and aesthetic reasons.
In modern homes and buildings, the heating function of fireplaces has been relatively unimportant because more effective alternative heating systems (e.g., centrally located forced air furnaces) are in widespread use. However, there are some situations in which the heating function of fireplaces is of significant importance. For example, in some small vacation cottages fireplaces are the only means available for providing heat.
Recent shortages of energy and the escalating costs of fuel have caused increased attention to be focused upon apparatus and methods for improving the heating efficiency of fireplaces, whether located in small buildings or cottages having no central heating system, or located in homes or other buildings having a central heating system.
Heat transmission from fireplaces can be divided into three categories:
1. Conduction PA1 2. Convection PA1 3. Radiation
With a typical open-front fireplace equipped with a cast iron fireplace grate, there is very little useful heat gain by conduction. Further, there is essentially no heat gain due to convection. In fact, room air is actually drawn into the fireplace and exhausted up the chimney during the time that a fire is burning in a fireplace. Consequently, there is actually a net loss of warm air from a room when a fire is burning in such a fireplace. For these reasons, substantially all of the heating effect of an open-front fireplace equipped with conventional cast iron or wrought iron grate results from radiant heat. This radiation travels through the room air, but has virtually no effect in warming the air as the radiation passes through the air. However, when radiant heat strikes a solid object such as a person, it does warm the object. It is extremely difficult to be precise in measuring the radiant heat output from fireplaces. However, it can generally be noted that the radiation of heat from a fire will vary considerably depending upon the type of fuel used, the size and temperature of the bed of coals, the distance between the fire and the object being heated by radiation, etc.
In an effort to reduce the loss of warm room air through open-front fireplaces, glass door closures have been used. These have the effect of substantially reducing, although not elminating, the loss of room air through the chimney. Since the glass door closure becomes hot, some minor convective heat output is generated by air currents within the room which come in contact with the room side of the glass closure. However, these convective heat gains tend to be offset by a decrease in the amount of heat radiated from the fire. This decrease is a result of radiant heat being reflected by the glass back into the fireplace.
In the interest of improving the heating efficiency of open-front fireplaces, tubular fireplace grates have been developed which provide heat to the room in which the fireplace is located in the form of convective heat output through the tubes. One popular design of such a tubular fireplace grate is shown in U.S. Design Pat. No. 228,728.
More recently, products have begun to appear on the market which combine the desirable features of a tubular or ducted heat-exchanging fireplace grate with a glass door closure. Combination products of this type reduce room air losses through the fireplace while increasing convective heat output through the tubes and from the face of the glass door closure. Such combinations of glass doors and grates have improved heating efficiencies, but their design has been accompanied by certain structural problems due to uneven expansion and contraction of the various component parts during heating and cooling, uneven heat distribution, and other problems.